Wash and the cucumbers. Then, slice the cucumbers very thinly. You can also use food processor with the thin slicer attachment, or the slicer opening on a cheese grater. Set aside.
Make the dressing by placing the vinegar, sugar, salt, and pepper in a serving bowl and whisking until the sugar dissolves completely.
Add the sliced cucumbers, red onion rings, and chopped dill, if using, and toss well. Marinate for 5 or more minutes and serve.